Not applicable.
Not applicable.
This invention relates generally to mini-cryo coolers and more particularly to an improved adiabatic micro-cryostat system for use in an infrared seeker in a missile.
In missile applications, it is often desirable to use an infrared seeker with infrared detectors to guide weapons to a target, particularly ground-to-air and air-to-air missiles. Infrared-sensitive devices, including infrared detectors, are preferably operated at temperatures as low as 80xc2x0 Kelvin and lower. At such low temperatures, infrared detectors operate most effectively and have increased sensitivity with increased signal-to-noise ratio. Because infrared detectors are typically installed on aircraft, missiles, and other mobile devices and because the detector itself is often gimbal-mounted for tracking objects, the cooling apparatus must not only provide low temperatures but must be relatively small in mass and size and must be able to operate in varied attitudes. A cryostat, which operates as an open cycle cryo-cooler, is an apparatus which provides a localized low-temperature environment in which operations or measurements may be carried out under controlled temperature conditions. Cryostats are used to provide cooling of infrared detectors in guided missiles, for example, where detectors and associated electronic components are often crowded into a small containment package. Cryostats are also used in superconductor systems where controlled very low temperatures are required for superconductive activity and in medical cryo instrumentation.
It has been found that a cryostat based on the Joule-Thomson effect can often meet these requirements. Although in some modem applications, where large imaging focal plane arrays are involved which require significant cooling rates, even a large Joule-Thomson cryostat cannot meet the requirements. A Joule-Thomson cryostat is a cooling device that uses a valve (known in the art as a xe2x80x9cJoule-Thomson valvexe2x80x9d) through which a high pressure gas is allowed to expand via an irreversible throttling process, resulting in lowering of its temperature. The simplest form of a conventional Joule-Thomson cryostat typically has a fixed-size orifice in the heat exchanger at the cold end of the cryostat such that cooling by the cryostat was unregulated. The input pressure and internal gas flow dynamics established the flow parameters of the coolant through the cryostat. Alternatively, some cryostats have gas throttling valves which provide the ability to start cool-down with the maximum orifice size, thereby providing high rate gas flow and refrigeration for rapid cool-down. After cool-down is achieved, the orifice size is reduced by the valve for minimal gas flow rate and sustained cooling for the thermal load. In a Joule-Thomson cryostat a flow of high-pressure coolant gas, such as nitrogen or argon at, for example, 4000-6000 pounds per square inch, is throttled. The cooling upon gas expansion converts the coolant to a liquid state. The low temperature of the coolant is then used to cool the infrared detector and the expansion-cooled outgoing coolant is also used to cool the incoming coolant. Another method of attempting to reduce the cool down time is using a dual gas system where a first gas, such as argon is used as the initial gas to reduce the cooling time and then the system switches to a second gas such as nitrogen to sustain the cooling effect at a lower temperature.
System parameters including cool-down time, the amount of cryogen, cryogen storage pressure, pressure gradient through the system, engineering complexity of dual gas systems and physical dimensions of the open and closed cryo-system must be traded off to provide a suitable cryocooler design. In missile applications, it is desirable to minimize the cool-down time such that the infrared detector can begin to operate more quickly after the launch. Furthermore, it is desirable to increase the efficiency of the stored bottle gas to provide a longer time of cooling of the infrared detector. Still furthermore, it is desirable to provide alternative cooling systems for millimeter wave low noise amplifiers, cryo-cooled electronics, high temperature superconductor systems and medical cryo instrumentation at efficiencies close to the ones afforded by industrial liquid cryo gas producers.
In accordance with the present invention, a cooling apparatus includes a cryostat operating on a Joule-Thomson effect and an adiabatic chamber having an input and an output, the output connected to an input of the cryostat. With such an arrangement, an improved cooling apparatus is provided with improved efficiency over known Joule-Thompson cooling apparatus.
In accordance with another feature of the present invention the adiabatic chamber includes a microturbine. With such an arrangement, the size and volume of the cooling apparatus can be reduced.
In accordance with still another feature of the present invention, the microturbine includes a microgenerator to take out the mechanical energy from the gas used in the cooling apparatus. With such an arrangement, mechanical energy of the gas can be taken out of the cooling system to reduce the cool down time of the cooling apparatus.
In accordance with a further aspect of the present invention the adiabatic chamber includes a piezo-electric material, or alternatively, a MEM (micro elecro-mechanical) fabricated device such as a silicon forest or fabricated deformation surface with a built-in voltage generator to take out energy from the gas used in the cooling apparatus. With such an arrangement, the method of removal of the mechanical energy from the immediate vicinity of the cooling volume facilitates the reduction of the cool down time of the subject thermal load.